1. Field of the Invention
The present invention relates to a photosensitive matrix electronic sensor. It relates more particularly to X-ray radiation sensors. The object thereof is to improve the performance of sensors of this type.
2. Discussion of the Background
Radio logical image intensifier screens disposed opposite a detector and receiving X-ray radiation on another face are known in the field of X-ray electronic sensors. Scintillators are also known, in the field of nuclear medicine, for transforming gamma rays (or X rays) into visible radiation which can be detected by a detector. The detectors most commonly used are, in the field of radiology, cameras with target or assembled arrays of charge coupled devices (CCDs). In the field of nuclear medicine, use is also made of banks of photomultiplier tubes linked moreover to a barycentration electronic circuit. All these sensors with detectors incapable of directly detecting X rays are associated with scintillators tasked with transforming X rays into radiation in the approximately visible spectrum.
The material used to perform the transformation, the material of the scintillator, is normally gadolinium oxysulphide. The latter is used in the form of a thin deposit, typically of from 50 to 300 micrometers. This deposit consists of particles of this material which are bound together by a binder. The emission of visible light throughout the thickness, and in all directions, of this material causes a loss of sensitivity and a loss of resolving power of the detector, and hence of the sensor.
A proposal has already been made to deposit a film (plastic) containing gadolinium oxysulphide on a luminous image matrix detector, the latter consisting of a silicon integrated circuit.
Caesium iodide CsI, doped with thallium, in the form of needles, offers a beneficial alternative for greater luminous efficiency allied with a waveguide effect of the needles, whose cross sections have typical dimensions of from 3 to 6 micrometers. This material is conventionally used in radiological image intensifiers, by coating an input screen which generally consists of a domed aluminium foil. Embodiments are also known in which a wad of optical fibres is covered by such a material. The needles are oriented perpendicularly to the surface of the support which carries them. They only partially adjoin one another. They thus offer a porosity of 20 to 25%. These air-filled pores, associated with the favourable refractive index of CsI (1.78) afford channelling of the visible photons emitted in each needle and impart a higher sensitivity and resolving power.
However, difficulties in using CsI, as compared with gadolinium oxysulphide remain. One is aware, in particular, of the drawback that CsI hydrates rapidly in ambient air and customary humidities. This uptake of water has the effect of degrading the image obtained with the sensor. It causes a halo effect initially. This humidification then irreversibly degrades the needles with a consequent loss of luminous efficiency and resolving power of the sensor. It should be noted that this drawback is not encountered in radiological image intensifier tubes since the CsI is in the vacuum tube.
Moreover, although present in small quantities in the needles, thallium is highly toxic. The low mechanical fastness of CsI then causes dust and waste matter, the elimination of which must be scrupulously controlled. In certain cases, the passivation of the thallium-doped CsI is achieved by vaporizing a layer of aluminium on the surface of the scintillator.
On account of its low mechanical fastness, the CsI must be deposited on a rigid support. The bending of the support would in fact give rise to visible defects in the image. This support must moreover normally undergo, without deforming, a heat treatment to diffuse the thallium at a temperature of the order of 300xc2x0.
In radiological image intensifiers, the support is made of aluminium, sometimes allied with amorphous carbon, or even replaced with amorphous carbon on account of the very great resistance of this material.
Outside of the construction of image intensifier tubes, the depositing of caesium iodide on beryllium has been envisaged. However, this material has the drawback of being excessively expensive.
The object of the invention is to solve these problems by advocating the growing of a CsI layer on a base consisting of a machined graphite block, preferably having the particular feature of exhibiting low surface roughness. Preferably, in the invention the graphite used as a base has undergone, at its surface, a densification step so as to eliminate the natural porosity thereof related to the graphite. Moreover, this layer thus rendered denser is preferably then ground so as to impart a low roughness thereto. It has then been found that, when deposited in the gaseous phase, the CsI adopts an entirely beneficial growth: the needles are regularly spaced and the surface of the scintillator thus produced is almost flat despite the defects related to the roughness of the graphite.
If the base is not made to undergo the densification operation, differences in sensitivity result within the sensor produced. One can attempt to put this right. For example, if the graphite surface is striated (for example with parallel lines), the presence of these striations is recognized in the image obtained after operation of the sensor. It is possible, especially in the field of nuclear medicine, to correct the differences in sensitivity relating to the various locations through software processing. In an enhancement according to the invention, one limits the magnitude of this correction through the densification operation and/or the grinding operation.
In all cases, the presence of the graphite base affords the solution to the problems of differential expansion occurring during the diffusion of the thallium.
Graphite, such as understood within the present invention, is a material which differs from amorphous carbon in the sense that it has a porous physical structure, unlike amorphous carbon which is very dense. Graphite can be machined with metal tools, whereas amorphous carbon is almost only machinable with diamond-encrusted tools.
This is why, in the application envisaged here, namely the deposition of caesium iodide on a machined support intended to be placed in front of a matrix image detector, it proves to be especially beneficial to use a graphite block as support.
Graphite usually has a structure which is not only porous but also lamellar, thereby further facilitating its machining, unlike amorphous carbon, whose structure is essentially isotropic.
In principle graphite is obtained by compressing carbon powder at high temperature, whereas amorphous carbon results from decomposition in the gaseous phase (cracking) culminating in the growth of coatings of greater or lesser thickness on a starting support. It is therefore easier to produce machinable blocks from graphite, whereas it appears to be easier to produce amorphous carbon coatings on surfaces such as the domed surfaces of radiological image intensifier input screens.
The subject of the invention is therefore a photosensitive matrix electronic sensor comprising a matrix image detector surmounted by a scintillator for transforming high-frequency electromagnetic radiation, typically X-ray radiation, into low-frequency radiation, typically radiation in the visible domain, characterized in that the scintillator comprises a caesium iodide faceplate carried by a graphite base disposed on the side where the high-frequency radiation is received.
Its subject is also a process for fabricating a sensor, characterized in that
a graphite base is made, this base having to serve as support for a scintillator,
the graphite base is ground,
caesium iodide is deposited in the gaseous phase on the graphite base,
the caesium iodide deposition is doped with thallium,
a layer made of a synthetic resin is deposited under vacuum in the gaseous phase on the caesium iodide deposition,
a layer of liquid optical coupling resin is deposited on the synthetic resin layer,
a detector is applied flat against the liquid optical coupling resin layer.